Ochiai

Ochiai is a scientist and member of the Immortal Ship Committee who lived centuries before the main storyline and was the foremost researcher on the Gauna's behavior and biology. His experiments nearly lead to the destruction of Sidonia. After he is arrested, his knowledge of the Guana is placed into an auxiliary brain, which only the Immortal Ship Committee has full access too.

Ochihai is able to hide a copy of his conciseness in his laboratory before being arrested. Years later when Kunato decides to enter Ochihai's sealed lab, his body is taken over by Ochihai's consciousness.
(Source: Sidonia no Kishi Wikia)